Ultra high molecular weight polyethylene (UHMWPE) is very resistant to impact and scratches, is known for self-lubrication, has a high melting viscosity, and typically has a weight average molecular weight in excess of 1,000,000, as determined in accordance with ASTM D-4020 viscosity methods. UHMWPE is used for preparing bearing surfaces, such as biomedical implants, e.g., orthopaedic implants, such as tibial plateaus, patellar buttons of prosthetic knee implants, hip implants, etc., typically made by either machining the components from a solid block of UHMWPE or molded directly from native (virgin) UHMWPE powder.
It is known that a major problem with total joint implants prepared by conventional molding and/or machining is wear and surface damage of the article from generated UHMWPE particles (debris) that can function as abrasives to the implant under contact stress. Reduction of contact stress may minimize debris generation, and wear and surface damage.
International Application, WO 97/29895, published on Aug. 21, 1997 (Burstein and Li) teaches that according to Bartel, et al. Trans of the ASME, 107, 193-199 (1985) "contact stress is a function of the elastic modulus of the bearing material in a contact bearing joint such as a knee joint," and it is desirable to produce an UHMWPE material with the lowest possible modulus of elasticity, (modulus), which retains the desirable strength properties of conventional UHMWPE. An implant can be prepared by machining or molding by a process other than that described in the above mentioned Burstein and Li patent application.
A successful attempt of producing UHMWPE with a low modulus UHMWPE material of between 500 MPa and 900 MPa is achieved through the controlled mold-quench process of the Burstein and Li International Patent Application discussed above. The Burstein and Li application discloses an invention where UHMIWPE is molded from a powder and at the end of the elevated heat--pressure process step the molded article is subjected to a quenching operation to produce an UHMWPE product with a lowered modulus of elasticity
Furthermore, the Burstein and Li process requires careful monitoring of mold body temperatures for the production of fully compacted non-distorted components that do not display surface cavitation. The Burstein and Li process does not relate to the lowering the modulus of elasticity of UHMWPE articles which have been previously prepared and removed from a mold or machining device to a finished dimension because the very surfaces which would typically be quenched to obtain the benefits of reduced modulus (e.g. the articulating surfaces of a total knee joint tibia component) must be carefully cooled in the heat pressing process well before the conclusion of the formation process. A near dimension part is one that allows a small, e.g. 0.025 inches, amount of additional material to allow for a finished machine cut.
Other processes for treating UHMWPE articles, such as U.S. Pat. Nos. 5,621,070 to Howard and U.S. Pat. No. 5,030,402 to Zachariades teach heating UHMWPE articles at temperatures in excess of 200.degree. C. In the Howard patent, rapid cooling is taught to be deleterious to the formation of the desired product.
Thus a need exists to provide biomedical implants having a low modulus of elasticity, between 500 MPa and 900 MPa, from any high modulus preformed or prefabricated polymeric article without resorting to the sophisticated control requirements of the prior art.